Booster amplifier for cellular telephone cradles

ABSTRACT

A booster amplifier that can be coupled to virtually any cellular telephone handset is provided for use with virtually any cellular communications network. The power output of the booster amplifier is dynamically controlled based upon signals received from the handset to improve the quality and range of communications between the handset and the base stations of the cellular network without interfering with normal communications between the cellular network and other handsets.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to cellular telephone communications and cradle systems for cellular handsets that allow a customer to use the handset in a hands-free mode such as when driving a motor vehicle. More specifically, the present invention relates to an apparatus and a process to dynamically control the power output of a booster amplifier which can be used in combination with virtually any handset cradle.

II. Brief Description of the Prior Art

The use of a cellular telephone handset while driving creates certain risks. First, such use can distract the driver. Second, one driver's hand must be used to hold the phone to the ear and mouth, leaving the other hand to operate the motor vehicle. Using handsets in this way exposes the driver to sufficient risks that it has been outlawed in several jurisdictions.

Recognizing these risks, various manufacturers have begun to provide accessories that permit hands-free use of a cellular telephone handset while driving. Generally speaking, the solutions offered by these manufacturers take one of two forms. The first is a headset which includes an ear piece and a microphone that the driver can plug into and wear while talking. Use of such headsets allows the driver to operate the vehicle with both hands. The second is a cradle into which the handset is placed, that include microphone and speaker interfaces. The microphone and speaker again allow the driver to carry on a conversation and while using both hands to drive.

Cradle systems offer various advantages over headsets. First, a real issue with cellular telephones is the need to conserve the power drawn from the telephone's battery. Many cradles not only provide a separate power source, but also have the ability to recharge the battery of the handset. Second, cradle systems connect to an external antenna to increase range and signal quality.

An additional improvement can be made by amplifying the signals that arrive from and to the phone through the use of a bi-directional signal booster amplifier.

Most cellular telephones conserve battery power by reducing the power of the handset's transmitter to the lowest level that will still provide active effective communication. The base station controlling the cell in which the handset is located measures the strength of the incoming signal it receives from the handset. The base station then issues an instruction to the handset and the power output is adjusted to the lowest level adequate to maintain communications. If the signal from the handset is too low, the base station will instruct the handset to increase output power. Since handsets typically have a low maximum output power, they may not be able to transmit at a power output level sufficient to maintain communications. The result is a “dropped call”. Dropped calls are annoying to the handset user and expensive for the cellular telephone network operator. This problem is particularly acute in cities where buildings can block the signal path. This same problem can occur when the user of the handset is traveling at high speeds through rolling countryside.

The prior art includes efforts to solve this problem by boosting RF power. A number of methods of boosting power when using a cradle have been described in the prior art. The prior art cradle based, power boosting techniques will be discussed in the context of the AMPS cellular telephone network used in North America. In the AMPS system there are three classes of cellular telephones. Class I telephones operate at a maximum power of 6 dBW. Class II telephones operate at a maximum power of 2 dBW. Class III telephones operate at a maximum of −2 dBW. Most handsets operate as Class III telephones to conserve battery power.

The first prior art technique to boost power of the Class III telephones is to provide the cradle with a second RF power amplifier. When the handset is placed in the cradle, the cradle's amplifier is activated so that the handset and the cradle work together as a Class I device. The amplifier in the cradle boosts the maximum RF power output to about 3.0 watts and thus provides a 7 dB advantage over a 0.6 watt handset. Such a cradle arrangement is shown in U.S. Pat. No. 5,457,814, to Markku Myrskog et al dated Oct. 10, 1995. The power boost system discussed in the Myrskog et al patent does not have universal application and a specific handset must be used with the cradle. It does not have gain control circuitry, nor coupling loss compensation. It does not disclose a universal boost system.

The second prior art technique to boost power involves providing the handset with a second RF power amplifier. The handset senses when it is placed in the cradle and activates the second amplifier. This changes the handset from a Class III device operating at 0.6 watts to a Class I device operating at 3.0 watts. This technique is discussed in U.S. Pat. No. 4,636,741 to James E. Mitzlaff dated Jan. 13, 1987. Again, a special handset is required and the cradle cannot be used to boost the power of any handset.

The third prior art technique is, perhaps, the simplest and least expensive of the four. When the handset is placed in the cradle, the handset increases its power to the maximum permitted for Class III operation. At the same time, the handset power control circuitry is disabled so the power output is not modulated in response to control signals issued by the base station to the handset. This can lead to significant interference with other handsets.

A fourth technique is disclosed in U.S. Pat. No. 6,029,074 to David R. Irvin dated Feb. 22, 2000. This patent discloses a handset which is set up to operate as a Class II telephone. It has control logic that allows it to modulate its power output in response to power attenuation signals it receives from the base station of the cellular systems. The control logic is also designed so that when the handset is not in the cradle, the power output cannot exceed a predetermined maximum value even if the attenuation signal received from the base station calls for a higher output. Thus, the handset acts very much like a Class III telephone when not in the cradle with the power output capped at −2 dBW and like a Class II telephone when in the cradle with a maximum power output of 2 dBW or about 1.6 W.

Those working in the field understand that generating a signal with higher power will reduce the number of dropped calls. Simply boosting the power, however, creates other problems such as interference with other cellular telephones and interference with normal cellular network operations. While telephones such as those disclosed in U.S. Pat. No. 6,029,074 would address some of these problems to a limited degree, such solutions still limit the dynamic range of the handset. Interference with other handsets and normal network operations remain a possibility. Other problems associated with use of a cradle are simply not addressed. These include the inherent loss of RF power between the handset's antenna and the cradle's coupling device and the loss in signal strength due to the length of cable from the cradle to the external antenna. Also, the AMPS system is not the only type of network used. A properly designed cellular booster amplifier is needed for CDMA, TDMA, PCS, GSM and iDEN network applications as well. The booster amplifier must be designed to work with analog, spread spectrum and other digital networks. There is also a need to provide a system that can accommodate many different kinds of handsets, even those that do not have the control logic of the type discussed above or internal amplifiers that permit the handset to operate as a Class I or Class II device.

A technique to control the gain of booster amplifiers is described in U.S. Pat. No. 6,175,748 B1 to Aboukhalil et al., dated Jan. 16, 2001, whereby the output of a booster is determined by a input level determining device determining the handset's output power, and compared to a level from a level determining device at the booster's output. The two levels are compared, resulting in a difference level, which is used as a reference to control a variable gain element. The level determining circuitry includes a step voltage circuit that produces a number of step voltages, which serves as a reference, for the gain adjustment of the booster. As a result, using pre-defined discrete step voltages to control the booster gain makes the attached handset power control function appear non-linear to the base station that it is communicating with. Thus there is a danger that the booster, under these circumstances, can go into compression when the handset is requested by the base station to go to high output power state, causing distortion and interference to other users on the network.

Another technique is described in U.S. Pat. No. 6,230,031 B1, whereby the signal losses between the mobile handset transceiver is compensated for by the adjustment of a bank of manual switches on the booster. This requires prior knowledge or a measurement of the cable loss between the mobile transceiver handset and the booster amplifier to be made. This design does not mitigate concerns of interference or minimizing the amplifier distortion when exposed to high handset output power.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a booster amplifier that enhances the performance of a cellular telephone while operating a motor vehicle.

Another object of the present invention is to provide a booster amplifier system that interfaces with a wide variety of cellular telephone handset or cradle that addresses the inherent loss of RF power between the handset's antenna and the cradle's RF coupling device.

Still another object of the present invention is to provide a booster amplifier to increase the range of the cellular telephone and reduce the number of dropped calls working in conjunction with a variety of cellular network types including AMPS, CDMA, TDMA, PCS, GSM and iDEN networks.

A further object of the present invention is to provide a booster whose maximum output power is leveled. The level at which this occurs is set below the transmit path output compression point of the booster power amplifier, ensuring that the booster amplifier's transmit path is highly linear and thus minimizes it's output distortion.

Another object of the present invention is to provide a booster amplifier that provides a gain on the booster amplifier's transmit path whose value is a function of the leveling set point of the booster amplifier and the handset's transmit RF output power. The gain setting is dynamically determined from the amplifier output power and the amplifiers leveling point. No low level or digital control signals are required from the handset to set the booster amplifier's transmit gain. The amplifier is self-calibrating and no manual settings are required.

A further object of the present invention is that the booster amplifier's transmit path gain and will not increase during any given power-on period. The gain will only be reset to its maximum value when the power is cycled off and on. The gain of the booster amplifier is a monotonically decreasing function from its leveling point.

Another object of the present invention is that the booster amplifier does not disable or interfere with the handsets own dynamic power control. The booster amplifier power output linearly mirrors the handsets gain control dynamics and will linearly track the handset in whatever steps the operating mobile network commands the handset to provide.

Another object of the present invention is that the gain of the receive and transmit paths are not the same. The transmit path gain of the booster is self-adjusting, while the receive path gain is factory set to a fixed level, enough to compensate for the RF coupling losses in mobile installations. Mobile booster amplifiers with high receive path gain can cause the handset to hand-off incorrectly while moving through its cellular network.

These and other objects are achieved by providing a booster amplifier designed to operate with all types of RF coupled cradles and/or handsets. The booster amplifier can also be connected to a cradle specially designed to work with a particular style of handsets. The booster amplifier will boost the handset's RF signal and is designed so that its power output is controlled by the closed-loop power control of the handset. Thus, when the handset receives a power control signal from the network's base station, the booster amplifier's output varies accordingly. The present invention incorporates a method for separating the transmit and receive circuitry, thus minimizing cross-talk interference between the transmit and receive paths. The transmit and receive circuits are separated from each other by their own respective ground planes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the preferred embodiment of the present invention.

FIG. 2 is a block diagram of the booster amplifier of the present invention.

FIG. 3 is a graph showing the transfer function of the booster amplifier showing the range of gain the booster amplifier will adjust to.

FIG. 4 is a graph showing the transfer function of the booster amplifier at start-up.

FIG. 5 is a graph showing the optimized amplifier transfer function.

FIG. 6 is a table reproduced from the TIA/EIA-553-B Mobile Station—Base Station Compatibility Standard showing the power level to be generated by Class I, Class II and Class III mobile telephones in response to eight different control signals generated by the base station.

FIG. 7 is a table based upon the TIA/EIA-98-D specification showing the power limits of Class I, Class II and Class III mobile telephones.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is designed for connection to various types of cellular communications networks. As shown in FIG. 1, the invention includes a telephone handset 1 which can be mounted in a cradle 2. The cradle includes a speaker and microphone (not shown) so that when the handset 1 is placed in the cradle 2, it can be used in a hands-free mode.

The cradle 2 is coupled by a coaxial cable 4 to a booster amplifier 6. The booster amplifier 6 is coupled by a second coaxial cable 8 to an antenna 10. The cradle 2, booster amplifier 6 and antenna 10 will typically be permanently mounted to a motor vehicle.

The booster amplifier 6 is designed to be universal, i.e. accommodate many different kinds of handsets 1 and work with virtually any commercial cellular network. Various cradles 2 may be used with this system. The cradle may have a direct RF connection to the handset or it may be inductively coupled. The booster amplifier 6 will operate for a range of input power levels, so it can work with handsets 1 having maximum output power levels up to 28 dBm. The amplifier 6 is designed so that it can boost a signal to its maximum power limit within a range of input power spanning 30 mW (15 dBm) to 650 mW (28 dBm). The wide input range of the booster amplifier 6 also assists with variable loss due to coupling variations to the cradle 2, connectors, and cables 4 and 8 used as part of the system.

FIG. 2 is a block diagram showing the manner in which the booster amplifier 6 is constructed. The booster amplifier 6 has a connection 12 which is used to connect the cradle 2 to the booster amplifier 6. The booster amplifier 6 has a connection 14 to which the antenna 10 is connected. A duplexer 16 is provided on the cradle side and a duplexer 18 is provided on the antenna side. The duplexers 16 and 18 are provided so that two-way communication between the handset 1 and cradle 2 on the one hand and a cell tower base station via the antenna 10 on the other hand can occur. The booster amplifier has RF paths, a receive path and a transmit path. Another important aspect of the design of amplifier 6 is the separation of the transmit path from the receive path by a ground plane 11. This prevents cross-talk interference between the circuitry of the transmit path and the circuitry of the receive path.

As signals are received from the base station by the antenna 10, they pass through connection 14 and duplexer 18 to the receive path. These signals then follow a path through a low noise amplifier 20, a fixed attenuator 22, the duplexer 16 and the connection 12 to the cradle 2 and handset 1. The attenuator provides a modest attenuation to set receive path gain. Received signals can include either analog or digital voice communication signals. Such signals also include control signals such as signals from the base station instructing the handset 1 to modify its power output.

The transmitted RF signal generated by the handset 1 passes through the connection 12 and duplexer 16. They then pass through the variable gain element 28, the driver amplifier 30, the power amplifier 32, and a directional coupler 34 before passing through the duplexer 18 and connection 14 for transmission by the antenna 10 to a cell system base station.

An important aspect of the invention relates to the manner in which the variable gain element 28 is controlled. The booster amplifier 6 includes a gain controller 36. The gain controller 36 receives input from output directional coupler 34 and the associated detector 40. Detector 40 senses voltages at the directional coupler 34 on the output side of variable gain element 28. Logic associated with the gain controller 36 processes the signal received from the detector 40 to control the variable gain element 28.

The gain controller 36 has a gain processor circuit 58, which incorporates a sample and hold function that will retain the gain setting derived from the gain limiter circuitry 54. Signals are delivered to the gain processor circuit via gain limiter circuitry 54 and by the detector 40. The gain processor 58 adapts the gain of the amplifier to the signal measured by its sample and hold circuit function. The gain controller 36 also has a limiter circuit 54 that monitors the output of the power amplifier 32. Specifically, limiter circuit 54 receives signals from detector 40 and uses these signals to limit the maximum power output of the booster amplifier 6. This serves to prevent intermodulation distortion and to keep the maximum output power within regulatory limits. Circuit 56 receives signals from both the gain processor 58 and the limiter 54 and allows the limiter 54 to override the gain processor 58 so as to limit the maximum output power. Logic circuit 56 sends signals to the variable attenuator element 28 to control the gain provided by the booster amplifier.

As set forth in the background of the invention, cell system base stations continually send control signals to handsets operating with the range of coverage of the base station. These control signals are used by the handset to control the power output of the handset 1. When the apparatus of the present invention is used, these control signals are received by the antenna 10 and pass through the booster amplifier 6 to the cradle 2 and handset 1. In response to these control signals, the handset adjusts the power of its output signals. As output signals of the handset 1 pass back through the booster amplifier 6, the output power of the amplifier is sensed by the directional coupler 34 and detector 40. The variable gain element 28, is adjusted to the desired level based upon instructions received from the gain controller 36. The signals then pass through the amplifier, directional coupler and duplexer and out through the antenna 10.

With the foregoing discussion in mind, the manner in which the booster amplifier 6 boosts the signals will be discussed in greater detail with reference to FIGS. 3-5. As reflected in FIGS. 3-5, the gain controller 36 of the booster amplifier 6 performs several important functions.

FIG. 3 illustrates the booster amplifier gain transfer function. As stated earlier, the gain of the booster amplifier 6 is variable, and the gain controller reduces the gain of the booster amplifier 6 depending on the output level at the detector 40. This ensures that the booster amplifier 6 always operates in the network compatibility region of the graph. This process is shown in detail in FIGS. 4 and 5. As shown in FIG. 4, when the booster amplifier 6 is powered up, it operates at maximum gain to accommodate low power handsets 1 or handsets with poor coupling into the cradle 2. The gain controller 36 detects the maximum output power of the booster amplifier 6 and dynamically adjusts the variable gain element 28, so as to maximize operation of the booster amplifier 6 in the network compatibility region. This allows the handset 1 to operate with its full dynamic power control range, while achieving up to the maximum power output level of the booster amplifier 6 when required to reach a cellular base station.

FIG. 5 is a graph showing the manner in which the power transfer function of the booster amplifier 6 is optimized. The amplified RF signal is received at the directional coupler 34, sensed by detector 40 and supplied to the gain processor 58 of the gain controller 36. If the output power would exceed the maximum output power limit, the gain controller 36 will reduce the total gain of the amplifier utilizing the variable gain element 28. This reduction in gain is maintained by the gain processor 58, and even if the handset reduces its output power, the gain will be constant.

The gain controller 36 will not increase the gain until the booster amplifier 6 is reset with a power-on cycle event. Whenever the booster amplifier 6 is reset, the gain is reset to provide a maximum output power of booster amplifier. This is desirable because the conditions presented to the booster amplifier 6 from the handset 1 are unpredictable. The user may switch handsets or the handset 1 may no longer be seated properly within the cradle 2.

The foregoing arrangement is superior to the use of a simple amplifier with a fixed gain that provides only limited output control. Those systems have the effect of reducing the dynamic range that the handset can control its output power within.

With the foregoing in mind, the operation of the preferred embodiment will be discussed in connection with an 800 MHz AMPS Class III handset 1. The maximum output power of a typical Class III handset 1 is 0.6 W. The present invention can be used to make the handset 1 operate either as a Class I handset with a maximum power output of 4.0 W or a Class II handset with a maximum power output of 1.6 W.

In an AMPS network, the step size specified is 4 dB. FIG. 6 is reproduced from the TIA/EIA-553-B Mobile Station—Base Station Compatibility Standard for AMPS cellular networks. In such a network, the base station sends out signals representative of 8 different power levels (numbered 0-7 in the FIG. 6). The table also shows the power levels Class I, Class II and Class III handsets operate at in response to such signals. As indicated in the table, all three classes of handsets operate at the same power level when the base station sends a code 2 through a code 7. For example, when a code 2 is sent all three classes of handsets operate at 28 dBm. However, when a code 1 is issued, a Class III handset continues to operate at 28 dBm while Class I and Class II handsets operate at 32 dBm. Similarly, when a code 0 is issued, Class III handsets operate at 28 dBm, Class II handsets at 32 dBm and Class I handsets at 36 dBm.

When a handset 1 first registers with the cellular network, it will transmit at maximum power (i.e. 0.6 W for a Class III handset). When the handset 1 is coupled to the cradle 2, the gain controller 36 booster amplifier 6 will set the gain of variable gain element 28 according to the power level at which the handset 1 is operating. When the base station sends the first power control command instructing the handset to reduce power (by 4 dB), the output power level of the amplifier will be 4 dB lower, as it is operating in its network compatibility region (constant gain region).

CDMA cellular networks differ from AMPS networks with respect to power control. In CDMA networks, power control is performed using two different methods—open-looped estimation performed by the handset and close-loop correction involving both the base station and the handset. When the handset first registers with the network, it uses open-loop power control. The handset bases its transmit power level on the received signal strength from the base station. Later, closed-loop power control commences. The base station sends control signals every 1.25 ms to the handset. These signals may instruct the handset to increase power, decrease power or keep the power level constant. The power increments are not always the same as is the case in AMPS networks. In CDMA networks the increments may be 1.0 dB, 0.5 dB or 0.25 dB. The lower and upper limits of the power ranges for Class I, Class II and Class III handsets used in a CDMA network are set forth in FIG. 7.

The CDMA closed-loop power control mechanism is more precise and offers quicker response times than the AMPS system. The requirements of the CDMA system are readily handled by the present invention. Since booster amplifier 6 has a linear gain characteristic, the finer power increments of the CDMA network are preserved at the output of the booster amplifier 6. While the booster amplifier 6 may not have set the optimum gain at start-up, this does not have a negative effect on the network because the gain controller 36 is always active and reacts quickly so that the closed-loop power control is accurately maintained. Further, the full range of power control remains available.

Various advantages are provided by the present invention whether used with an AMPS, CDMA, GSM and TDMA networks, or some other type of network. A principle advantage is that the present invention can be used with any Class III handsets and all possible transmit power levels from such handsets. The booster amplifier 6 reacts to the power level coming from the handset 1 to conform to the network constraints and enhances the power control dynamic range. The booster amplifier 6 limits its output power and creates no intermodulation distortion by operating in its linear region.

Another significant advantage is that use of a handset 1 with the booster amplifier 6 within the service area will not interfere with other handsets used by other network users. The booster amplifier 6 will not always be transmitting at its full output power, but rather at the level determined by the network's closed-loop power control system.

These and other advantages are all achieved through the use of the present invention which, of course, may be modified without deviating from its true scope. The foregoing discussion has been provided to meet the disclosure requirements of the patent laws. It is not intended to be limiting. The scope of the invention is, of course, defined by the following claims. 

1. A RF power boost control system on an amplifier's transmit path for enhancing communications between a handset and a network base station of a cellular network comprising: a booster amplifier having, a. at least one variable gain element controlled by a gain controller; b. at least one amplifier which cooperates with the variable gain element to boost the power of an input signal generated by the handset to provide an output signal to the network base station; and c. a sensor that sends messages to the gain controller indicative of the power level of the signal generated by the handset after it is boosted by the at least one amplifier, said gain controller dynamically adjusting the power of the output signal up to a predetermined power leveling range.
 2. A RF power boost control system on an amplifier's transmit path for enhancing communications between a handset and a network base station of a cellular network as in claim 1 wherein, the booster amplifier has, a. a receive side used to process and deliver signals received from a base station to a handset, said receive side including at least one low noise amplifier; b. a transmit side used to process and deliver signals from the handset to the base station, said transmit side having a variable attenuator element, at least one amplifier, and an output sensor for sensing the power of signals transmitted; c. a ground plane separating the transmit side from the receive side.
 3. A RF power boost control system on an amplifier's transmit path for enhancing communications between a handset and a network base station of a cellular network as in claim 1 wherein, said transmit side having, a gain controller circuitry and, a variable gain element controlled by said gain controller controlling the booster system's transmit gain, an output directional coupler and a detector which generates a signal indicative of the output power to the gain controller, and said gain controller capable of processing signals to control the variable gain element on the transmit side.
 4. A method of operating the power boost system of claim 1 with the step of, controlling the maximum output power of the transmit path by using the gain controller circuitry from which a signal input to the booster's transmit path cannot be further amplified, the level at which this occurs is set below the transmit path output compression point of the at least one amplifier, ensuring that the booster amplifier's transmit path is highly linear thus minimizing it's output distortion.
 5. A method of operating the power boost system of claim 1 with the step of, limiting the maximum power of the output signal to a predetermined threshold.
 6. A method of operating the power boost system of claim 1 with the step of, determining the leveling set point of the booster amplifier gain for the transmit path dynamically as a function of the amplifier output and the amplifiers leveling point.
 7. A method of operating the power boost system of claim 1 with the step of, following the handset gain control dynamics in the booster amplifier to linearly track the handset output such that the handset's transmit power control range is not decreased.
 8. A method of operating the power boost system of claim 1 with the step of, monotonically decreasing the transmit path gain during any given power-on cycle period until it is power cycled off and then re-initialized on another power-on cycle event. 